Field of the Invention
The invention refers to a control unit with harmonic command variables or set point signals and to a method for operating a control device with harmonic command variables.
Description of the Prior Art
Conventional control methods are tailored to control problems with constant or only slowly changing command variables in which the value of a controlled process variable affected by a disturbance is kept close to a predetermined set point, or is updated as close as possible to a varying set point. Some applications (e.g. micromechanical rotation rate sensors for analysis of a Coriolis force) provide excitation of an oscillator with its resonance frequency and with an oscillation amplitude defined by a control loop. In this process, a controller controls the force signal generated by it so that the difference between a predetermined harmonic set point signal and a measured oscillator movement vanishes.
In this process, a measurement signal which reflects the movement of an oscillator along a direction of excitation typically is at first fed to a demodulator. The demodulator multiplies the measurement signal with a harmonic signal of angular frequency ωd that corresponds to the resonance angular frequency ω0 of the oscillator. Control is itself performed with a set point signal in the baseband being constant or, in any case, independent of the resonance angular frequency. The output signal of the controller is then re-modulated in a modulator on a harmonic signal with an angular frequency ωm, which corresponds to the resonance frequency ω0 of the oscillator. The modulation product is then compared with the predetermined set signal. The difference between the two signals finally controls an actuator, which imposes a force to the oscillator based on the controller signal so that the oscillator oscillates according to the predetermined set oscillation. As control is performed within baseband, a low pass filter filters the frequency conversion products, in particular at the double resonance frequency. The bandwidth of the controller and, hence, its reaction speed to changes of the deflection is limited by such process.
A control with a harmonic set point signal in the frequency band of the resonance angular frequency ω0 of the oscillator is described in not yet published German patent application DE 102010055631.9.
In many applications the oscillator oscillates with a decaying amplitude after deactivation of the drive. In the control activated during a decay phase of the oscillator, the activation time depends on the phase and amplitude difference between the decaying oscillation and the set point signal.